When Chandler and Monica learned they couldn't concieve, they paid a visit to Bill and Colleen to learn about Owen's adoption. Chandler bumped into Owen on the way to the bathroom and accidentally revealed that Owen was adopted before his parents could tell him. Owen was initially upset at this revelation ("I'm adopted?!") but was even more horrified to learn that Santa wasn't real ("He isn't?!"). This resulted in Monica and Chandler making a hasty escape. Bill, Colleen and Owen do not make any more appearances.
In the uncut version of the episode, Bill reveals that he's 8 when Monica makes a mistake of saying he's 12. During the credits, Monica suggests he may have been adopted from Russia because he's tall for his age.